Good points everyone. I already do have a link going to one of my sites on every freesite. Sometimes it's the same domain as the freesite is on, other times it's a different site on my network. I do this both for traffic and SEO as well. Of course, these being my sites, I have nothing to worry about. I do sometimes submit freesites in niches where I don't have hubs/sites of my own, hence the idea to rent out those spots.
I see your point about traffic UW, but I think some people do actually want links for PR alone. I have noticed that most of my freesites that made it into the recent update got a PR4, I think some people would find that alone attractive enough
I guess there is quite a management toll involved here. I might do this only for trusted webmasters, people that I know from the boards. It also makes me think that per-month is better than lifetime, in the sense that it will mean the webmaster has to stay in touch with me, and I with their site. I will also make very strict rules about the pages that I link to.